Polarizing beam-splitters are used for many applications to physically separate or combine polarized light in two orthogonal polarizations, by transmitting light in one polarization state and reflecting light in the other polarization state. The useful properties of a PBS are often described as the operational wavelength range or bandwidth, the angular field or the operational angular range, the average extinction ratios (the ratio of the light intensities of desired polarization over the undesired polarization) in both transmitted and reflected beams, and the transmittance and reflectance of the desired polarizations. Polarizers are a special case of polarizing beam-splitters in which only one beam is required, either the transmitted or reflected beam. Detailed descriptions of existing thin film interference based polarizers and polarizing beam-splitters and their applications can be found in the paper “Visible broadband, wide-angle, thin-film multilayer polarizing beam splitter” by Li and Dobrowolski in Applied Optics, Vol. 35, pages 2221-2225 (here referred to as reference D1), the paper “High-performance Thin Film Polarizing Beam Splitter Operating at Angles Greater than the Critical Angle” by Li and Dobrowolski in Applied Optics Vol. 39, pages 2754-2771 (here referred to as reference D2) and the book chapter “Optical Coatings for Displays” by Li in the book “Optical Interference Coatings”, Springer 2003, edited N. Kaiser and H. K. Pulker (here referred to as reference D3).
One example of an application for polarizing beam-splitters is in three-dimensional stereoscopic projection displays. A polarizing beam-splitter is used for combining left- and right-eye images with orthogonal polarizations for three-dimensional (3D) displays in various configurations as disclosed in the U.S. patent application Ser. Nos. 11/770,247 and 11/780,910 and 12/045,119 by Boothroyd and which are incorporated herein as references D4, D5 and D6, respectively. FIG. 1 shows a representative arrangement for the use of a polarizing beam-splitter in such 3D projection displays. The PBS separates the unpolarized incident light into first and second orthogonally polarized light beams which are directed to two reflective micro-display panels 1 and 2, respectively. The micro-display panels can be liquid crystal on silicon (LCOS) or digital light processing (DLP) micromirror arrays with waveplates, for example. Left- and right-eye images are displayed on the two panels and are encoded by polarization. For “on” pixels, the reflected image light beams change polarization from the first to the second polarization and vice versa. The orthogonally polarized image light beams are then combined by the polarizing beam-splitter into a single beam as shown in FIG. 1 and the combined image light is directed to a projection lens.
For 3D projection displays, as well as many other applications, it is often required that the PBS operates over a broadband of wavelengths, a wide angular field, has high average extinction ratios or high contrast in both transmitted and reflected beams, and has high transmittance or reflectance in the desired polarizations. For example in a 3D projection display, a broadband wavelength range allows the PBS to operate for all colors in the visible spectrum to make simple two-panel 3D projectors; a wide angular field allows large aperture optics to be used to collect more light and improve image quality; high average extinction ratios allow high contrast images to be formed with low-cross talk between left- and right-eye images. Although there are different types of polarizers or polarizing beam-splitters available today, each of them has limitations either due to poor performance, large device size, or high manufacturing cost, that prevents them from being widely used in applications such as 3D projection displays.
Thin film interference based polarizers or PBSs are the most commonly used polarizers and PBSs because of their design flexibility and relative ease of manufacture. The first prior-art broadband thin film polarizers are the so-called conventional MacNeille polarizers. These are based on the principles of thin film interference as well as the Brewster angle. A conventional MacNeille polarizer consists of a quarterwave thin film multilayer coating having low and high refractive indices sandwiched between two prism substrates. A MacNeille polarizer operates centered at the Brewster angle and for an angular field of only a few degrees in air. Furthermore, it has a high average extinction ratio for the transmitted beam, but not for the reflected beam even at the designed Brewster angle (typically lower than 20:1). The conventional MacNeille polarizer is often used as a polarizer, rather than a PBS, as its name indicates. Its narrow angular field and low average extinction ratio in reflection prevent it from being used in many applications, such as 3D projection displays in which a wide angular field of ±12° (measured in air) and high average extinction ratios of at least 100:1 in both the reflected and transmitted beams are required.
The second type of prior-art broadband thin film polarizers and polarizing beam-splitters are modified MacNeille polarizers or PBSs. Modified MacNeille polarizers still operate centered at the Brewster angle and all operation angles are below the critical angle defined by the refractive index of the substrate and the lowest refractive index in the coating. Modified MacNeille polarizers are an improved version of the conventional MacNeille polarizer where the layer thicknesses have been optimized and result in non-quarterwave PBS coating designs. To further improve the performance, additional intermediate refractive index layers are introduced as described in the reference D1. FIG. 2 shows the calculated performance of a PBS example from reference D1 with an improved angular field of ±6.1° in air for the visible spectral region and an average extinction ratio about 100:1 in both transmitted and reflected beams. Although it has better performance than conventional MacNeille polarizers and other modified MacNeille polarizers, its performance still does not meet the requirement, especially in the angular field, for many applications including 3D projection displays.
The third type of thin film broadband polarizing beam-splitter was disclosed in U.S. Pat. No. 5,912,762 and in reference D2, this device is referred to as the Li Li polarizing beam-splitter or Li Li PBS in the present application. Unlike the conventional and the modified MacNeille polarizers, the Li Li PBS is based on thin film interference and frustrated total internal reflection. It has broadband wavelength operation, a wide angular field, high average extinction ratios for both transmitted and reflected beams and high transmittance or reflectance in the desired polarizations. The disadvantages of the Li Li PBS are, however, that the incident angles or operational angles are above the critical angle and larger than those of conventional or modified MacNeille polarizers because of the requirement of frustrated total internal reflection. This results in larger prism sizes and consequently large size optical systems and higher cost which is not desirable in many applications. Furthermore, the Li Li PBS preferably requires the use of high index prism substrates (preferably with index higher than 1.70) that need optical contacting due to the unavailability of suitable high refractive index optical glues, this also results in high manufacturing cost.